Wow, yesterday I took my car to a dealer to fix the same exact problem!
I got MKIV with 29.1 update in my 04 X5. Worked fine for 3 month (after update to 29.1) and on Feb 24th, 2008 my Nav got lost, placed me 10-15 miles away from where I was. Went into a service menu and looked at GPS Status which was showing: "Receiver Error" I had no green dots anymore in the right upper corner where the sattelites should be. Almanac: No Visual Sattelites: 8

I think there is a bug in that update and we are going to see more posts with this issue. Something is wrong with BMW's NAV.

BTW here is my original post: http://www.xoutpost.com/x5-e53-forum/...receiver+error

Let me know if your problem fixes itself arturocorso.